New features of the pulmonary circulation have recently been described in certain types of pulmonary hypertension, in particular (1) a reduction in non- or partially-muscular arteries of the lung up to 140 micron m in diameter, and (2) the distribution and severity of the increased muscularity. When lung vessels are distended by injection and assessed quantitatiely, the features of the normal and diseased circulation at various ages can be compared precisely. The object is to analyse quantitatively structural changes associated with three animal models of pulmonary hypertension each representing a different mechanism, to follow the evolution of the changes and their reversibility, and to relate the structural findings to changes in respiratory and circulatory function. PROJECT I is concerned with establishing the structural changes in the lung of the rat after (1) exposure to low oxygen tensions and (2) after feeding Crotalaria spectabilis: and in the pig (3) after aortopulmonary shunting; to quantify by light and electron microscopy the degree and site of increased muscularity of arteries and veins at all levels, and by electron microscopy the seeming 'loss' of vessels noted by light microscopy. PROJECT II is concerned with elucidating pulmonary hypertension in the pig induced by an aortopulmonary shunt before and after lung growth is completed. In PROJECT III cardiopulmonary function will be studied in the three animal models both during development of pulmonary hypertension and when it is established since, in at least one of these 'models', the circulation has reduced capacity. The response of this abnormal circulation to drugs and exercise will be followed as it models certain human types. PROJECT IV is a quantitative morphometric study of human lung disease and congenital heart disease associated with disturbance of the pulmonary circulation.